1. Field of the Invention
This invention relates to a method of, and apparatus for, stabilising the output of a mode-locked fibre ring laser, and to a mode-locked fibre ring laser.
2. Related Art
Mode-locked fibre ring lasers offer potentially the best quality (pulse width, pulse shape and time x bandwidth) optical source for producing solutions. A fibre laser typically needs to contain tens or even hundreds of meters of fibre for two reasons: to obtain sufficient gain from the rare-earth-doped fibre, and also to include all required intra-cavity components to enable controlled operation. The fundamental frequency of a laser cavity of this length is in the MHz region. Hence, to use a fibre laser as a source of optical pulses at GHz frequencies (the range of most interest to communications systems at present), it is necessary to achieve stable operation at very high harmonics of the fundamental cavity frequency. This requires active control of the length of the fibre laser. Mode-locking at high harmonics, and the susceptibility of fibre to its environment, lead to output noise and instability, unless unwanted cavity modes can be suppressed and the cavity length kept at the right value. Harmonically mode-locked lasers behave quite differently from fundamentally mode-locked lasers; and, generally speaking, their output is inherently noisier. This is due to supermode competitions associated with harmonic mode-locking. The reasons for this will now be explained.
Thus, in an Nth harmonic mode-locked laser, the cavity mode spacing is f.sub.c, and the modulation frequency f.sub.m =Nf.sub.c. An axial mode is not locked to its nearest neighbors, but to the ones N axial modes apart on each side. All the modes within the linewidth are thus grouped into N sets, or into N so-called supermodes. Every supermode forms a separate mode-locking solution, and satisfies all the modulation and lasing conditions. As a result, all these modes can oscillate at the same time more or less independently and compete against each other, unless one of them, for some reason, saturates the gain and thus suppresses the others. The energy shift among these supermodes, and the relative phase slides between them, readily leads to pulse amplitude fluctuations and even to missing of some pulses. Reflections (e.g. from intracavity elements) into the gain medium (erbium fibre in this case) affects the mode competition, as they form standing waves in the lasing medium, and thus cause spatial hole boring. In simple erbium fibre ring laser operations, sporadic noise bursts, which are related to supermode beatings in the laser's radio frequency (RF) spectrum, are observed. Letting only one supermode oscillate is the key to the stable operation of such lasers.
An effective way to suppress the unwanted supermodes is (see "Harmonically mode-locked fiber ring laser with an internal Fabry-Perot stabilizer for solution transmission"-Optics Letters, vol. 18, No. 2 Jan. 1993) to insert a high finesse Fabry-Perotetalon into the fibre cavity, the etalon having a free spectral range which is exactly equal to the pulse repetition rate. The etalon acts as a very narrow band-pass comb filter, and so picks up only one supermode. The intra-cavity etalon needs careful control, and a separate cavity length adjustment is still required.